In digital communications systems, digital data is translated into analog radio signals for transmission, and received radio signals are processed to extract the digital data. In digital communications systems, a mixture of analog and digital components is often used. Typically, digital components are used in the data processing portions of these systems, while analog components are used in the radio frequency portions.
Over the years, the boundary between the digital portions and analog portions has trended toward the analog ends of these systems. Functions formerly performed using analog components have been taken over by digital components. Digital components can offer over advantages over analog components, such as repeatability, stability, and cost. As a particular example, in complex baseband processing, it can be difficult to match gain, phase, filter responses, and other parameters between the inphase channel and quadrature channel when analog components are used.
Unfortunately, the bandwidth of signals that can be processed by a digital system is limited by the sampling rate. Higher sample rates can allow higher bandwidths, but also require higher processing rates. Thus, as sample rates increase, faster digital signal processors and/or increased numbers of processing components are often required. In some situations, the desired sample rates are less than the available processing, limiting the bandwidth (and in some situations, the communications data rate) that can be provided.
A particular area where a limitation in sample rate or processing speed is felt is in performing digital upconversion of signals, for example, as used in a transmitter. In transmission, digital data is typically encoded into a modulated signal, converted into analog form, and then upconverted (typically in several steps) to the desired transmission frequency. Some of this upconversion can be performed digitally, however this can be a processing intensive operation.